Method and apparatus for dynamic power management in an execution unit using pipeline wave flow control

ABSTRACT

Power is conserved by dynamically applying clocks to execution units in a pipeline of a microprocessor. A clock to an execution unit is applied only when an instruction to the execution unit is valid. At other times when the execution unit needs not to be operational, the clock is not applied to the execution unit. In a preferred embodiment of the invention, a dynamic clock-control unit is used to provide a control signal to a local clock buffer providing a local clock to an execution unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims the benefit of thefiling date of, U.S. patent application Ser. No.. 10/042,082 entitledMETHOD AND APPARATUS FOR DYNAMIC POWER MANAGEMENT IN AN EXECUTION UNITUSING PIPELINE WAVE FLOW CONTROL, filed Jan. 7,2002 now U.S. Pat. No.7,137,013.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a clock scheme in a microprocessorand, more particularly, to dynamic power management in an execution unitof a microprocessor using pipeline wave flow control.

2. Description of the Related Art

Today's microprocessors designed in CMOS technology dissipate more andmore power. Thus, cooling microprocessors, as well as supplyingsufficient power, becomes a challenge. In CMOS technology, powerdissipation is due to charging and discharging capacitances introducedby a following stage of circuits and the connecting wires. Typically,power dissipation ‘P’ is proportional to the frequency ‘f’ of switchingthe capacitive load of all circuits and is also proportional to thesquare of the supply voltage ‘V_(t)’. Thus, Pαf*V_(t)**2.

In addition, as processor speeds increase, execution units within aprocessor must implement deeper pipelines in order to meet the smallercycle times. This represents an increase in the amount of power neededdue to register clocking and switching. However, execution unitstypically do not operate at 100% utilization, but operate at 10-20%utilization. Thus, much of this power usage is unnecessary. That is tosay, for at least 80% of the time when there are no instructions flowingin the pipelines, power is still being consumed due to register clockingand switching.

Therefore, there is a need for controlling a pipeline of an executionunit in a microprocessor such that when no valid instructions are beingexecuted, the clock to each unused stage in the pipeline is dynamicallycontrolled so that no switching occurs and power is conserved.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, a microprocessor isconfigured for executing at least one instruction. The microprocessorhas a main processor clock. A first stage having one or more storagecomponents is configured for storing operand data of the at least oneinstruction. The first stage is clocked by at least a first clockderived from the main processor clock. A first combinatorial logic isconnected to the first stage for receiving the operand data from thefirst stage and is configured for processing the operand data andgenerating first output data. The first clock is operational only duringa first period of time when the operand data is processed by the firstcombinatorial logic. A second stage of one or more storage components isconfigured for storing the first output data. The second stage isclocked by at least a second clock derived from the main processorclock. A second combinatorial logic is connected to the second stage forreceiving the first output data from the second stage and is configuredfor processing the first output data and generating second output data.The second clock is operational only during a second period of time whenthe first output data is processed by the second combinatorial logic.

In another embodiment of the present invention, a method is provided fordynamically reducing power consumption in a microprocessor configuredfor executing at least an instruction. The microprocessor has a mainprocessor clock. Operand data is stored in a first stage of one or morestorage components residing in the microprocessor. The first stage isclocked by at least a first clock derived from the main processor clock.The operand data is transmitted from the first stage to a firstcombinatorial logic residing in the microprocessor. The first clock isoperational only during a first period of time when the operand data isprocessed by the first combinatorial logic. The operand data isprocessed in the first combinatorial logic. First output data isgenerated from the first combinatorial logic. The first output data isstored in a second stage of one or more storage components residing inthe microprocessor. The second stage is clocked by at least a secondclock derived from the main processor clock. The first output data istransmitted from the second stage to a second combinatorial logicresiding in the microprocessor. The second clock is operational onlyduring a second period of time when the first output data is processedby the second combinatorial logic. The first output data is processed inthe second combinatorial logic. Second output data is generated from thesecond combinatorial logic. Power consumption is reduced in themicroprocessor by dynamically controlling the first and second clocks.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 depicts a high-level block diagram showing one embodiment of thepresent invention in a processor using master-slave latch design;

FIG. 2 depicts a block diagram showing a gate-level embodiment of thepresent invention in a processor using master-slave latch design; and

FIG. 3 depicts a timing diagram showing an operation of one embodimentof the present invention as shown in FIG. 1.

DETAILED DESCRIPTION

The principles of the present invention and their advantages are bestunderstood by referring to the illustrated operations of embodimentsdepicted in FIGS. 1-3.

In FIG. 1, a reference numeral 100 designates a high-level block diagramshowing one embodiment of the present invention in a processor usingmaster-slave latch design. Although the block diagram 100 isspecifically applicable to a processor using master-slave latch design,the present invention is also applicable to any other latch designs.

The block diagram 100 includes a dataflow pipeline 102. Preferably, thedataflow pipeline 102 is implemented in an execution unit of amicroprocessor. The dataflow pipeline 102 includes an array 104connected to master-slave latches 106, 108, and 110 for storing the dataread from array 104. The array 104 is a storage component within theprocessor for storing operands used in instructions to be executed inthe dataflow pipeline 102. The array 104 is connected to the latches106, 108, and 110 for providing up to three operands to the latches 106,108, and 110. Without departing from the true spirit of the presentinvention, any number of latches may be used, depending on the number ofoperands to be processed. The latches 106, 108, and 110 are connected toa first combinatorial logic 112 for storing the operand(s) beforeproviding the operand(s) to the first combinatorial logic 112. When oneor more operands are provided to the first combinatorial logic 112, acomputation is performed therein. The first combinatorial logic 112 isconnected to a latch 114 for generating a first output data of thecomputation and providing the first output data to the latch 114.

The latch 114 is connected to a second combinatorial logic 116 forstoring the first output data before providing the first output data tothe second combinatorial logic 116. The second combinatorial logic 116is connected to the latch 118 for generating a second output data andproviding the second output data to the latch 118. The latch 118 isconnected to a third combinatorial logic 120 for storing the secondoutput data before providing the second output data to the thirdcombinatorial logic 120. The third combinatorial logic 120 is connectedto a latch 122 for generating a third output data and providing thethird output data to the latch 122. The latch 122 is connected to thearray 104 for storing a fourth output data before providing the fourthoutput data to the array 104.

It is noted that the dataflow pipeline 102 is not limited to thisspecific configuration. For example, the latches 106, 108, 110, 114,118, and 122 each may be replaced with a register, which comprises aplurality of latches. Also, additional latch stages may also be added tocreate deeper pipelines.

The latch 106 is shown to be connected to a first local clock buffer(LCB) 124 for receiving a C1 clock and a C2 clock from a first LCB 124.Although, for the sake of simplicity, the latches 108 and 110 are notshown to be connected to the first LCB 124, both the latches 108 and 110are similarly connected to the first LCB 124 for receiving the C1 and C2clocks from the first LCB 124. The C1 clock is directed to the masterlatch stages of the latches 106, 108, and 110, where as the C2 clock isdirected to the slave latch stages of the latches 106, 108, and 110. Thelatch 114 is connected to a second LCB 126 for receiving the C1 clockand the C2 clock from the second LCB 126. Likewise, the latches 118 and122 are connected to a third LCB 128 and a fourth LCB 130, respectively,for receiving the C1 clock and the C2 clock. The number of LCBs employedin the block diagram 100 may vary depending on the number of cyclesrequired by instructions processed in the dataflow pipeline 102.

The LCBs 124, 126, 128, and 130 receive a main processor clock (notshown) from which all other clocks, including the C1 and C2 clocks, arederived. The LCBs 124, 126, 128, and 130 are shown to be connected to adynamic clock-control unit 132 for receiving information on whether todisable the clocks to a corresponding latch or latches. For example, theLCB 124 is connected the dynamic clock-control unit 132 for receivinginformation on whether to disable the clocks to the latches 106, 108,and 110. Preferably, the dynamic clock-control unit 132 generates aninstruction-valid control bit to dynamically control the clockgeneration performed by the LCBs 124, 126, 128, and 130. For example, bytaking the instruction-valid control bit as it travels through thepipelines of a processor, one can enable the clocks to the correspondingpipeline stage as the instruction progresses through the pipelines. Ifthere are no valid instructions for a given cycle, or if the instructionis invalidated through flush mechanisms or load misses, then thissignals clock-control drivers (not shown) within the dynamicclock-control unit 132 to stop the clocks. If the instruction is valid,then this triggers the clock-control drivers to turn the clocks back onagain. The implementation of the instruction-valid control bit may takemany different forms, depending on a particular configuration of thecircuitry in the dynamic clock-control unit 132. For example, the LCBs124, 126, 128, and 130 may be configured to be turned on when theinstruction-valid control bit is asserted.

The benefit of this implementation is the power conserved, when no validinstructions are flowing through the pipelines of a processor. Theamount of power so conserved may be significant, because validinstructions may not always be flowing through a particular stage of apipeline. It is noted that different types of latches may be used toimplement the present invention, although a particular type of latchesis used herein to describe the present invention more clearly.

Referring now to FIG. 2, a block diagram 200 is shown to depict acontrol logic 201 connected to a master local clock buffer (LCBC1) 202and a slave local clock buffer (LCBC2) 204. The LCBC1 202 and the LCBC2204 are included in an LCB 205 to provide the C1 and C2 clocks,respectively. The LCB 205 is equivalent to the LCBs 124, 126, 128, and130 of FIG. 1. Preferably, the control logic 201 is part of the dynamicclock control unit 132 of FIG. 1 and is responsible for driving one ofthe LCBs 124, 16, 128, and 130 of FIG. 1. The control logic 201dynamically controls the LCB 205 depending on the validity of aninstruction under process. Also, the LCBC1 202 and LCBC2 204 receive aMESH clock, i.e., a main processor clock from which all other clocks arederived.

The control logic 201 includes two clock-control drivers 206 and 208configured for generating the aforementioned instruction-valid controlbit. The LCBC1 202 is connected to the clock-control driver 206 forreceiving a c1_stop_ctl signal from the clock-control driver 206.Similarly, the LCBC2 204 is connected to the clock-control driver 208for receiving a c2_stop_ctl signal from the clock-control driver 208.The c1_stop_ctl and c2_stop_ctl signals represent stop control signalsfor C1 and C2 clocks, respectively.

The clock-control drivers 206 and 208 each has a control input, a selectinput, and a phase hold input. The phase hold input of the clock-controldriver 206 is connected to an AND gate 210 for receiving an outputsignal from the AND gate 210. The AND gate 210 is connected to a latch212 for receiving a functional clock-stop request signal from the latch212. The AND gate 210 is also connected to a chicken switch 214 forreceiving an allow_dpm signal. The chicken switch 214 comprises latchesimplemented for functional failsafe overrides. The allow_dpm signalindicates an allow dynamic power management control signal. The AND gate210 receives two other signals HID0 and lbist_en_b. The HID0 signalindicates a bit signal from a hardware implementation dependentregister. The lbist_en_b signal indicates whether the system iscurrently in a logic built-in self-test (LBIST) mode. In theconfiguration provided in FIG. 2, the lbist_en_b signal is asserted whenthe system does not perform an LBIST test. Optionally, the AND gate 210can have additional input(s) (not shown) for other types of testing suchas an autonomous built-in self-test (ABIST) mode.

The phase hold input of the clock-control driver 208 is connected to anAND gate 216 for receiving an output signal from the AND gate 216. TheAND gate 216 is connected to the AND gate 210 for receiving an outputsignal from the AND gate 210. The AND gate 216 also receives aCOP_scan_sel_b signal, which is asserted when the system is not in scanmode. COP stands for common on-board processor. “COP” signals, such asthe COP_scan_sel_b signal, are derived from a “pervasive logic” on thechip (i.e., a logic responsible for clock-control of the chip amongother things).

The control input of the clock-control driver 206 receives a COP stopcontrol (COP_stop_ctl) signal. The select input of the clock-controldriver 206 receives a power_down signal. Both of these inputs toclock-control driver 206 are used to stop the clocks regardless of thevalue of the instruction-valid control bit. The phase hold input of theclock-control driver 206 receives a stop_c1_req signal, which indicatesa request to stop C1 clock signal.

Similarly, the control input of the clock-control driver 208 receivesCOP C2 phase stop control (COP_stopc2_ctl) signal. The select input ofthe clock-control driver 208 receives the power_down signal. Both ofthese inputs to clock-control driver 208 are used to stop the clocksregardless of the value of the instruction-valid control bit. The phasehold input of the clock-control driver 208 receives a stop_c2_reqsignal, which indicates a request to stop C2 clock signal.

The LCBC1 202 and the LCBC2 204 are connected to a latch 218 forproviding the C1 and C2 clocks, respectively, to the latch 218.Likewise, the LCBC1 202 and the LCBC2 204 are connected to a latch 220for providing the C1 and C2 clocks, respectively, to the latch 220. Asindicated in FIG. 2, there may be additional latches (not shown) otherthan the latches 218 and 220. These additional latches would besimilarly connected to the LCBC1 202 and the LCBC2 204. The number oflatches receiving the C1 and C2 clocks varies, depending on theconfiguration of a particular stage of a pipeline. For example, in FIG.1, there are three latches 106, 108, and 110 receiving the C1 and C2clocks from the LCB 124. In all the subsequent stages of the dataflowpipeline 102, there is one latch per each stage such as the latches 114,118, and 122. Therefore, depending on the configuration of a particularstage of a pipeline in which the LCB 205 is located, the number oflatches required therein may vary.

The AND gate 210 allows the functional clock-stop request signal to passthrough, provided that the chicken switch 214 for that unit is set, theHID0 bit is set, and the system does not perform an LBIST test. The ANDgate 216 transfers the stop_c1_req signal to the phase hold input of theclock-control driver 208, provided that the system is not in scan mode.Thus, scan chains can always be shifted regardless of the value of thefunctional stop request signal.

In FIG. 3, a timing diagram 300 is shown to provide an operation of oneembodiment of the invention as shown in FIG. 2. The free-running C1 andC2 clocks are shown to represent the C1 and C2 clocks without thecontrol logic 201. A functional input to the latch 212 is shown toprovide an input signal to the latch 212. The stop_c1_req andstop_c2_req signals represent the same signals as shown in FIG. 2.Likewise, the c1_stop₁₃ ctl and c2_stop_ctl signals represent the samesignals as shown in FIG. 2. The C1 clock pulse at a target latch, suchas the latches 218 and 220, has a clock pulse 302. The clock pulse 302represents one clock pulse of the free-running C1 clock, during whichthe c1_stop_ctl signal is deasserted. Similarly, the C2 clock pulse attarget latch has a clock pulse 304. The clock pulse represents one clockpulse of the free-running C2 clock, during which the c2_stop_ctl signalis deasserted. The latches 218 and 220 as shown in FIG. 2 (and possiblysome other latches not shown in FIG. 2) are considered target latches.

It will be understood from the foregoing description that variousmodifications and changes may be made in the preferred embodiment of thepresent invention without departing from its true spirit. Thisdescription is intended for purposes of illustration only and should notbe construed in a limiting sense. The scope of this invention should belimited only by the language of the following claims.

1. A method for dynamically reducing power consumption in amicroprocessor configured for executing at least an instruction, themicroprocessor having a main processor clock, the method comprising thesteps of: storing operand data in a first stage of one or more storagecomponents residing in the microprocessor, the first stage being clockedby at least a first clock derived from the main processor clock;transmitting the operand data from the first stage to a firstcombinatorial logic residing in the microprocessor, wherein the firstclock is operational only during a first period of time when the operanddata is processed by the first combinatorial logic; processing theoperand data in the first combinatorial logic; generating first outputdata from the first combinatorial logic; storing the first output datain a second stage of one or more storage components residing in themicroprocessor, the second stage being clocked by at least a secondclock derived from the main processor clock; transmitting the firstoutput data from the second stage to a second combinatorial logicresiding in the microprocessor, wherein the second clock is operationalonly during a second period of time when the first output data isprocessed by the second combinatorial logic; processing the first outputdata in the second combinatorial logic; generating second output datafrom the second combinatorial logic; generating at least twoinstruction-valid control bits; in response to the instruction-validcontrol bits, reducing power consumption in the microprocessor bydynamically controlling the first and second clocks by selectivelydisabling at least one local clock buffer to prevent switching of thefirst clock or the second clock; generating a scan mode signal, whereinin response to the scan mode signal, the instruction-valid control bitsenable the first clock and the second clock; and disabling the firstclock by a first instruction-valid control bit in response to a firststop control signal and disabling the second clock by a secondinstruction-valid control bit in response to a second stop controlsignal.
 2. The method of claim 1, further comprising the steps of:transmitting at least the first clock from a first local clock buffer tothe first stage only during the first period of time; generating thesecond clock from a second local clock buffer; and transmitting at leastthe second clock from the second local clock buffer to the second stageonly during the second period of time.
 3. The method of claim 1, furthercomprising the steps of: transmitting at least the first clock from afirst local clock buffer to the first stage only during the first periodof time; generating the second clock by a second local clock buffer;transmitting at least the second clock from the second local clockbuffer to the second stage only during the second period of time;generating a first control signal; transmitting the first control signalfrom the dynamic clock-control unit to at least the first local clockbuffer; and enabling the first clock signal by the first control signalto be operational only during the first period of time.
 4. The method ofclaim 1, further comprising the steps of: storing the operand data in anintegrated storage component residing in the microprocessor;transmitting the operand data from the integrated storage component tothe first stage; and transmitting the second output data from the secondcombinatorial logic to the integrated storage component.
 5. The methodof claim 1, further comprising the steps of: transmitting at least thefirst clock from a first local clock buffer to the first stage onlyduring the first period of time; generating the second clock from asecond local clock buffer; transmitting at least the second clock fromthe second local clock buffer to the second stage only during the secondperiod of time; storing the operand data in an integrated storagecomponent residing in the microprocessor; transmitting the operand datafrom the integrated storage component to the first stage; andtransmitting the second output data from the second combinatorial logicto the integrated storage component.
 6. The method of claim 1, furthercomprising the steps of: transmitting at least the first clock from afirst local clock buffer to the first stage only during the first periodof time; generating the second clock from a second local clock buffer;transmitting at least the second clock from the second local clockbuffer to the second stage only during the second period of time;generating a first control signal by a dynamic clock-control unitresiding in the microprocessor; transmitting the first control signalfrom the dynamic clock-control unit to at least the first local clockbuffer; using the first control signal to enable the first clock signalto be operational only during the first period of time; storing theoperand data in an integrated storage component residing in themicroprocessor; transmitting the operand data from the integratedstorage component to the first stage; and transmitting the second outputdata from the second combinatorial logic to the integrated storagecomponent.